Old man chopped up nut (6)
I believe the answer is:
almond
'nut' is the definition.
(almond is a type of nut)
'old man chopped up' is the wordplay.
'chopped up' indicates an anagram.
'old'+'man'='oldman'
'oldman' is an anagram of 'ALMOND'.
(Other definitions for almond that I've seen before include "Oval-shaped edible seed" , "one of the nuts" , "Nut often used in cake icing (6)." , "Marzipan-producing nut" , "Oval-shaped nut" .)